The present invention relates to a sharpness control circuit for a color television receiver.
The appearance evaluation of the picture image in terms of the edge characteristic of objects is usually identified by the term "sharpness." The more clearly defined the line which separates dark areas from lighter ones, the greater the sharpness of the picture. Picture quality can be improved by over-emphasizing the brightness transitions, thus improving the apparent contrast and sharpness of the picture.
Many of the presently known television receivers provide a manual preference control for selecting the degree of picture sharpness, the sharpness being changed by peaking the video frequency response around 2.5 MHz. In one known sharpness control circuit, the manual preference control switch introduced one of a plurality of fixed value capacitors in a filter for a video signal to establish a certain frequency response for the video signal. This type of control circuit has been successful in its purpose which is to permit the selection of the degree of sharpness which is subjectively preferable to the viewer. However, this and other known sharpness control circuits do have features which limit their more widespread use and effectiveness. A fixed amount of peaking added is generally not optimum for all the different television stations serving an area. The optimum subjective setting is a function of the receiver location, antenna and the fine tuning of the television receiver. The additional peaking requirements are also dependent on the varying amount of crispening introduced by studio personnel who subjectively judge the quality of the picture sent out for transmission. Furthermore, the known sharpness control circuits cannot distinguish signal from noise, and therefore, while building up the sharp edges in the picture, the circuit also accentuates the granular noise. This appears as snow in the picture. The noise accentuation effect is most apparent for weak channels and for such channels, the reproduced picture may appear too noisy and less appealing.